The invention relates in general to communication systems and more specifically to synchronization in a multiple-carrier communication system.
Due to advantages over other modulation techniques, communication systems utilizing multiple-carrier signals are currently being implemented for a variety of applications. Communications systems using Orthogonal Frequency Division Multiplexing (OFDM) techniques are gaining acceptance for applications such as broadcast television, mobile wireless and fixed wireless, including wireless local loop (WLL) applications. OFDM modulation techniques provide high data rate transmission over hostile channels with a system having a relatively low complexity. A typical wireless transmission channel subjects a transmitted signal to multi-path dispersion, resulting in numerous versions of the signal arriving at the receiver at different times. The transmitted signal is reflected and refracted through multiple transmission paths having different characteristics. The resulting interference between the versions of the signal causes inter-symbol interference (ISI) of the transmitted data. OFDM techniques typically employ a guard time between symbols to reduce ISI. Also, since OFDM utilizes multiple-carrier signals transmitted through the transmission channel, frequency-selective fading impacts a smaller portion of the transmitted data than is the case with single-carrier systems. OFDM techniques simplify the complexity of multiple-carrier receivers by using a rectangular-shaped sub-carrier for generating the multiple-carrier signals.
The performance in an OFDM system, however, is highly correlated to the synchronization between the transmitter and receiver. Small errors in frequency or timing can greatly reduce the performance of the system. Many conventional synchronization techniques are limited in that the received data must be demodulated and analyzed in order to correct for synchronization errors and as a result require significant overhead of time and processing power.
In an exemplary embodiment of the invention, a loss of synchronization between a receiver and a transmitter is detected by observing energy of a pilot signal within a guard frequency band. At least two pilot signals are transmitted adjacent to a guard band where one pilot is transmitted immediately below a guard band and another is transmitted immediately above another guard band. Deviations in synchronization between the transmitter and the receiver result in at least some of the energy of one of the pilot signals being received within one of the guard bands. By observing the energy within the guard bands, a loss of synchronization between the receiver and the transmitter can be detected. Resources reserved for synchronization are employed to regain synchronization after a loss of synchronization is detected.
In the exemplary embodiment, the determination that a loss of synchronization has occurred is based on three parameters. A phase detector is used to determine phase error or frequency error between successive waveforms. A relationship between the frequency and the phase of a plurality of pilot signals is examined to determine if the waveform timing error exceeds a timing error threshold a number of times within a given time interval. The ratio of the guard band energy determines a frequency error. If two or more of the synchronization monitoring methods indicate a loss of synchronization, a receiver is determined to have lost synchronization with the transmitter.